1. Field of the Invention
The present invention relates to a method for determining a drop in the capacity of a sodium-sulfur battery, including a case where a failure of an electric cell occurs.
2. Description of Related Art
A sodium-sulfur battery may be cited as one of electric power storage devices which are expected to see a wider use in various application fields, such as load leveling, measures against a momentary drop in power, or compensation for fluctuations in the power generated by a renewable energy generating device. The operating cycle of the sodium-sulfur battery is such that a cycle which consists of a discharge and a charge is repeated with an idle period between the repeated cycles in the application of, for example, the load leveling.
The sodium-sulfur battery is generally configured by connecting a plurality of electric cells in series to form a string, and then connecting a plurality of the strings in parallel to form a block. Further, a plurality of the blocks is connected in series to form a module, and then a plurality of the modules is connected in series to complete the sodium-sulfur battery.
Failure of the sodium-sulfur battery to exhibit its expected performance leads to a problem in the aforesaid applications, such as the load leveling, measures against a momentary drop in power, and the compensation for fluctuations in power. Hence, it is necessary to monitor the sodium-sulfur battery for a failure or abnormality.
Hitherto, in such a sodium-sulfur battery, voltage measurement has been carried out on each block, which is formed of a plurality of strings connected in parallel, each of which is comprised of electric cells connected in series, when the voltage of the battery is stable during a period of rest from the completion of a discharge to the start of a charge. If a block indicating a relative drop in voltage is found, then it has been determined that an electric cell or cells in the block have failed and therefore the (entire) sodium-sulfur battery has failed.
JP-A-2004-247319, for example, may be cited as a prior art.